Method for creating a uniform nip pressure utilizing a compliant pressure roller

ABSTRACT

The present invention provides a method for creating uniform pressure at a nip comprising the acts of providing a stationary inner core, pivotally mounting a plurality of shoes to the stationary inner core, creating a plurality of annular chambers wherein each of the shoes occupies one of the chambers. In addition, the method further provides filling each of the chambers with a magneto-rheological fluid, surrounding the plurality of chambers, shoes, and stationary inner core with a thin, metal, rotatable shell; detecting a deformation in the shell and changing a magnetic field in at least one of the chambers to change a viscosity of the magneto-rheological fluid to correct the deformation.

CROSS REFERENCE TO RELATED APPLICATION

This is a Divisional of application Ser. No. 10/795,010, filed Mar. 5,2004, now U.S. Pat. No. 7,258,654.

FIELD OF THE INVENTION

The present invention relates in general to pressure rollers, and inparticular, to a compliant pressure roller which adjusts to deformationin a thin outer shell.

BACKGROUND OF THE INVENTION

In preparing certain substrates it is important that at least onesurface have a smooth finish. Often this smooth finish requires veryhigh tolerances. Prior art solutions to the problem of creating a smoothfinish have not been completely satisfactory. One prior art solution hasbeen to use elastomeric rollers. Elastomeric rollers, however, cannot beground to the same high tolerance as metallic rollers.

Another prior art solution has used carrier webs having a smooth surfacefinish as a backing material for the substrate as it passes through thenip. A problem with this solution is that the carrier web usually mustbe discarded or recycled adding to the cost of the manufacturingprocess. Another problem is that the carrier web often wrinkles.

Using a roller having a metal sleeve with a smooth surface finish overan elastomeric backing is a possibility. This solution, however, alsohas drawbacks. The thin metal sleeve is subject to cracking afterrepeated use, especially along the margins.

One prior art solution is discussed in U.S. Pat. No. 5,146,664(Biondetti). The solution proposed is a series of hydraulic pistons.These hydraulic pistons, however, operate on a thick outer shell tocorrect for beam deflections in the roller. An apparatus as disclosedwould be expensive and not suitable for all applications.

For a patterned roller, a rapid pressure rise at a nip is important toforce material to the pattern. Metallic shells of small diameterinherently will create a small nip footprint, for a given nip load,which results in a higher pressure. In the formation of extrusion castweb materials the nip action improves the replication of the polished orpatterned roller surfaces.

In an ideal situation, rollers would be perfectly cylindrical and themolten resin would be uniformly distributed. In practice, neithercondition is achieved. An ability to locally adjust roller shape isdesirable to improve cross width nip loading. Consequently the webmaterial will have more consistent thickness and surface uniformity.Self adjusting rollers are utilized in paper manufacturing or webcalendaring operations which require high nip loads, but they haveminimal heat transfer capabilities.

It is desirable to have a roller capable of creating a smooth finish,with a small nip footprint, having an improved heat transfer capability,and capable of adjustment.

SUMMARY OF THE INVENTION

Briefly, according to one aspect of the present invention a compliantpressure roller for creating uniform pressure at a nip has a stationaryinner core. A plurality of shoes is pivotally mounted to the stationaryinner core. Non-magnetic dividers create a plurality of annular chambersand one shoe occupies one of the chambers. A magneto-rheological fluidfills each of the chambers and a plurality of magnetic field generatorschanges a magnetic field in each of the chambers. A thin, rotatableshell surrounds the plurality of chambers, shoes, and stationary innercore. Changes in the magnetic field generators causes changes in theresistance of the magneto-rheological fluid, which in turn exerts forceon the thin, rotatable shell, thus compensating for small deformities inthe shell.

A pressure roller, according to the present invention has a thin walledmetallic shell. The profile of the shell under load can be adjusted atvarious points along the rotation axis to provide a uniform contact nip.Regions are created along the axis of this roller in which the forceexerted on the inner surface of the shell can be varied to compensatefor forces that cause local shell deformation.

In an embodiment, a hydrodynamic force is created on the inner shell bythe shearing action of a magneto-restrictive fluid contained within thechambers created between the inner shell and the outer shell. Changes tomagnetic field affect the fluid viscosity. This generates a largerhydrodynamic force for a given rotation speed. The main componentsconsist of an internal cylindrical core, spring loaded pivot shoes, zoneseparation rings, magnetic field source, and thin walled external shell.

A thin shell on the roller provides finer axial profile adjustment andsuperior heat transfer capability, which is important for extrusioncasting systems. Improved heat transfer may allow for a smaller rollerdiameter which improves die to nip geometry and enables shorter meltcurtain lengths. This can improve melt curtain thickness non-uniformitydue to reduced neck-in. The roller can also be used on non-nipapplications in which the surface of the roller with respect to anotherobject needs to be adjustable.

The invention and its objects and advantages will become more apparentin the detailed description of the preferred embodiment presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric representation of an axially compliant pressureroller according to the present invention with a partial cross sectiontaken through the outer shell to expose internal components.

FIG. 2A is a cross section of the axially compliant pressure rollershown in FIG. 1 taken at the center of one of the internal flowchambers.

FIG. 2B is an enlarged view of the interface of the shoe and the outershell as noted in FIG. 2A.

FIG. 3 is a graph of radial deformation (UX) of the outer shell normalto the nip point for various internal pressure conditions.

FIG. 4 is a cross section of a nip formed between two rollers

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be directed in particular to elements formingpart of, or in cooperation more directly with the apparatus inaccordance with the present invention. It is to be understood thatelements not specifically shown or described may take various forms wellknown to those skilled in the art.

Referring now to FIG. 1 a compliant pressure roller is referred to ingeneral by numeral 10. Compliant pressure roller 10 is comprised ingeneral of a stationary inner core 12 and a plurality of shoes 14 whichare pivotally mounted to the stationary inner core 12. A series ofnon-magnetic dividers 16 create a plurality of annular chambers 18 andeach of the shoes 14 occupies one of the annular chambers 18.

Referring to FIGS. 1, 2A, and 2B, shoe 14, which is eccentricallymounted, is shown. One surface of shoe 14 is curved. A pivot point 15and spring loading assembly 17 are attached to shoe 14. A non-magnetic,metallic material is used in the construction of the shoe 14, but thepresent invention is not limited to this embodiment. The curvature ofthe face of the shoe 14 is slightly smaller than the curvature of theinner surface of the thin walled outer shell 24. This creates aconverging cross section at the interface between these components.

The compliant pressure roller 10 comprises of a non-rotating inner core12, which is the main support structure for the compliant pressureroller 10. A non-magnetic, metallic material is used in the constructionof the inner core, but the present invention is not limited to thisembodiment. The inner core 12 has a cylindrical form in which axialholes 19 have been provided. At least one of these holes is used tohouse the magnetic field generator 13. In the preferred embodiment onemagnetic field generator 13 is associated with each of the plurality ofshoes 14. This allows for local adjustments to the thin walled outershell 24. In an alternate embodiment a magnetic field generator 13 maybe located in each of the plurality of shoes 14 as shown in FIG. 2B.

Axial holes 19 are used for the circulation of heat transfer mediawithin the core. A series of pockets 26 are created in a radialdirection to serve as supports for the shoes 14. Seats on the inner core12 enable mounting of bearings 20 and fluid seals 22.

In operation, the hydrodynamic effect of a viscous fluid subject to theshear stress created by the relative velocity of the thin walled shellwith respect to the shoe, develops a pressure profile within theconverging section 11. This pressure acts on the thin walled shellcurved surface 25 and the curved surface 27 of the shoe. The pressureacting on the shoe results in a force normal to the curvature at thecenter of pressure. This force is resisted by the spring preloadingforce acting on the shoe 14. The pressure acting on the rotating thinwalled outer shell 24 creates an internal force on the shell. The netdifference in force acting on the shell from the internal hydrodynamicaction and the external nip force will result in a localized deformationof the thin walled shell in this region.

A thin walled shell of small shell diameter is possible with thisembodiment because the structural design of the shell is not dictated bybeam bending criteria or shell crushing criteria. The wall thickness ofthe shell can be significantly thinner because the surface of the shellsubjected to the external nip force is directly supported internally bythe pressure created by the interaction of the magneto-rheological fluid28 and the shoe 14.

The thin walled outer shell 24 is constrained with bearings 20 to rotateabout the inner core 12. The rotation of the shell can be imparted bythe friction force at the nip point 42, shown in FIG. 4, or with anexternal drive mechanism. Along the curved surface 25 of the thin walledshell, for a given convergent interface, relative velocity, and fluidviscosity create a uniform pressure. The annular chambers 18 inconjunction with the shoes 14, magneto-rheological fluid 28, and axiallyvariable magnetic field generator 13 can be subjected to variablehydrodynamic pressure forces by changing the viscosity of the fluid. Theability to exert axially variable pressure along the thin walled shellresults in localized deformation changes of small magnitude and at amuch higher frequency than possible by other prior art.

FIG. 3 shows the results of finite element calculations used to modelthe effect of the variable internal pressure capability of thisapparatus on the radial profile of the roller surface in the nip point.The dimensions of the shell can be represented in terms of the followingquantities; a flexural rigidity of approximately 1800 lb-in and a shellthickness to diameter ratio of 0.025. The flexural rigidity is definedas the quantity of the product of the material elastic modulus and theshell thickness cubed divided by the quantity of the product of aconstant value 12 and the quantity of the difference of 1 and Poisson'sratio squared. An average nip pressure of 250 psi, placed on the thinwalled outer shell 24 along a localized region parallel to the axis ofrotation, has been used in this calculation. The variable (UX) is theradial displacement in the x-direction, which is also normal to theapplied nip pressure region.

Curve 30 with diamond shaped markers represents the expected shelldeformation under nip load but without internal support. Curve 32 withtriangular shaped markers represents the effect of applying a localizedpressure, on an area equivalent to the curved surface of the shoe 14acting at the center of the shell with an average pressure of 50 psi.Curve 34 with rectangular shaped markers represents the positive effecton the radial deformation obtained by applying a gradient pressureprofile along the inner surface of the shell ranging from 15 psi to 20psi. Utilizing basic fluid dynamic principles it has been calculatedthat a pressure of approximately 30 psi can be developed in this regiongiven a fluid of viscosity of approximately 10 Pa-s sheared between theouter shell and the curved surface of the shoe with an average shearrate of 250 1/s.

FIG. 4 shows a cross sectional view of a typical two roller nip utilizedin the extrusion cast web formation. A compliant pressure roller 10 isloaded radially into the interface of the molten resin 52 and a secondroller 40. Utilizing a non-contacting deformation detector 50 such as alaser triangulation gage or an eddy current device, the resulting shellsurface deformation can be measured. This measurement data can beutilized to control internal loading conditions along the axis of theroller by sending a deformation signal 54 to microprocessor 56, whichalters the strength of one or more of the magnetic field generators 13.

In addition to the magneto-rheological fluid described previously, thisapparatus can accommodate other fluids without magneto-rheologicalproperties but which exhibit non-Newtonian characteristics (viscosity offluid is dependent on shear rate imposed). Localized pressure variationscan be created through adjustment of the gap between the outer shell andthe curved surface of the shoe. The average shear rate in this gap isproportional to the surface velocity of the shell divided by the gapheight. Non-Newtonian fluids exhibit a logarithmic relationship betweenviscosity and shear rate. External manipulation of the gap combined witha fluid with desirable shear sensitive properties provides an additionalmeans of creating localized pressure differences within each chamber.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention.

Parts List

-   10 compliant pressure roller-   11 converging section-   12 inner core-   13 magnetic field generator-   14 shoe-   15 pivot point-   16 non-metallic dividers-   17 spring loading assembly-   18 annular chambers-   19 axial holes-   20 bearings-   22 seals-   24 thin walled outer shell-   25 curved surface, shell-   26 pockets-   27 curved surface, shoe-   28 magneto-rheological fluid-   30 curve-   32 curve-   34 curve-   40 second roller-   42 nip-   50 deformation detector-   52 molten resin-   54 signal-   56 microprocessor

1. A method for creating uniform pressure at a nip comprising: providinga stationary inner core; pivotally mounting a plurality of shoes to saidstationary inner core; creating a plurality of annular chambers whereineach of said shoes occupies one of said chambers; filling each of saidchambers with a magneto-rheological fluid; surrounding said plurality ofchambers, shoes, and stationary inner core with a thin, metal, rotatableshell; detecting a deformation in said shell; and changing a magneticfield in at least one of said chambers to change a viscosity of saidmagneto-rheological fluid to correct said deformation.
 2. A method as inclaim 1 wherein a hydrodynamic force on said magneto-rheological fluidcreated by rotation of said shell compensates for deformation in saidshell.
 3. A method as in claim 1 wherein a magnetic field generator ismounted in each of said shoes.
 4. A method as in claim 1 wherein saidmagneto-rheological fluid is a heat transfer agent.
 5. A method as inclaim 1 comprising the additional step of spring loading said shoes tosaid stationary inner core.
 6. A method as in claim 1 wherein an eddycurrent detector detects deformation in said shell.
 7. A method as inclaim 1 wherein a laser detector detects deformation in said thin,rotatable shell.